Used system can be ok, but here are some things to think about:
Find out the history of the unit. When was the last time EACH of the filters, including the membrane was changed.
How long has the system been sitting around unused? Membranes should be used at least once a week.
More often than not, you'll have to start by replacing all the filters.
Sanitize the system before you install new filters
On a 10 yr old system assume that you'll have to at a minimum, lube the orings. But you'll probably need to replace the orings. Unless you know the brand of the housings (not the vendor who put their name on the system), your unlikely to be able to buy the orings from an RO vendor. You'll have to got to a gaskets/oring vendor locally with your orings and housings in hand.
If the system has clear housings, look them over very closely for cracks, chips, deep scratches. If you find any, replace the clear housing.
Last edited by BuckeyeFieldSupply; 05-04-2013 at 11:45 AM.